The present invention relates to a backlight system for a liquid crystal display apparatus, and more particularly to a thin and low-cost liquid crystal display apparatus using light emitting diodes as a backlight and efficiently irradiating light to a liquid crystal display.
In order to further improve an image quality of a liquid crystal display of a thin liquid crystal TV and further thin the display, light emitting diodes are used in some cases as a backlight, in place of a conventional cold cathode fluorescent lamp. As liquid crystal displays are becoming large in size, an LED backlight system is now at a transition stage from a side light type (disposing LED's on the side plane of a liquid crystal display) to a just-under light type (disposing LED's on the back plane of a liquid crystal display) having a short LED radiation distance, from the viewpoint of improving and uniformizing a screen luminance.
Although an LED is generally a point light source having a predetermined directivity, it is necessary for light emitted from LED's to be irradiated to a liquid crystal display as a uniform area light source.
Techniques of obtaining a uniform luminance in an LED backlight system are known as disclosed, for example, in JP-A-2007-42404, JP-A-2006-344409 and JP-A-2007-134224.
Techniques of reducing luminance irregularity, without increasing a thickness of a liquid crystal display the size of which is becoming large, are known as disclosed, for example, in JP-A-2007-134224.
JP-A-2007-42404 discloses techniques of uniformizing a luminance, in the side light type in which light from LED light sources is made to enter from the side plane of an optical guide plate disposed under a liquid crystal panel, by forming a prism as a reflection surface on the optical guide plate, and by gradually changing a prism inclination angle in accordance with a distance from an optical incidence surface.
JP-A-2006-344409 discloses techniques of improving luminance distribution, in a just-under type illumination apparatus for radiating illumination light from the back surface of a liquid crystal display, by forming a concentric concave/convex prism structure on an optical control plate for controlling a light emission direction of each LED light source.
JP-A-2007-134224 discloses techniques of reducing the luminance irregularity and thinning a liquid crystal display large in size, by disposing a just-under type area light source apparatus in such a manner that an optical axis at the highest luminance of LED is slanted relative to an optical guide plate.